The Range Extender means you can drive across United States without depending on the grid; the REX is a gas re-generator. The BimmerCode hack(s) makes it more enjoyable to actually go the distance. I now have a combined 140 miles per charge and ~2.5L gas reserve. There is a popular i3 Facebook Group where people log their long distance drives.

All of that said, my average commute is 10 miles so I'm purely electric about 95% of the time unless I forget to plugin. I got the REX because I thought the resell would be better in Texas.

The i3 has a "BEV" version that is much cheaper but more limited in its range; newer models are getting better tho.